This invention relates to chip-carrier substrates.
The invention is more particularly concerned with leadless chip-carrier substrates.
With the increasingly large scale of integrated electronic circuit chips the number of input and output connections that have to be made to the chip has correspondingly increased. This has resulted in a move away from dual in-line chip housings, in which two rows of connection pins are mounted on opposite sides of the chip housing, to leadless chip carriers. Leadless chip carriers usually comprise a square plate of ceramic, such as alumina, on which the chip is mounted. Electrical connection to the carrier is made by plated metal contact pads formed around each side of the carrier. The carrier is surface mounted on a printed-circuit board or substrate simply by placing the carrier on top of corresponding contact pads formed on the board or substrate and making electrical and mechanical connection by soldering. This arrangement is less cumbersome than that by which dual in-line housings are mounted on a board and allows for a greater density of input and output connections.
Disadvantages do, however, arise with leadless chip carriers because of the way in which they are connected to the substrate. Unlike dual in-line housings where connection is made through relatively flexible pins, the leadless carrier is rigidly joined to the substrate, lacking any ability to accommodate relative movement between the carrier and the substrate. If the carrier and the substrate are of materials having different coefficients of thermal expansion, change in temperature will cause differential thermal expansion between the two components. This leads to strain on the soldered connections which can, especially after repeated thermal cycling, cause failure of the connection, and in severe cases can cause the carrier to become detached from the substrate. Attempts have been made to match the thermal expansion properties of the carrier and substrate but, even when the two parts are of identical material it does not completely overcome the problem because the carrier and the substrate may not be at the same temperature. This is a particular problem where the assembly is repeatedly subjected to temperature differences as a result of the thermal energy that is dissipated by the semiconductor components in the carrier.
Various other arrangements have been proposed to reduce the effect of temperature differences on the mounting of leadless carriers but these have not yet proved successful in entirely overcoming the problem.